^ I think we can put an end to this digression by agreeing that music doesn't have objective quality but has fairly consistent effects on human beings, especially if they come from the same cultural background as each other, especially if they all come from the same cultural background as the music.
Things that cause music to have a consistent effect would be psychoacoustics. Harmonic and melodic intervals have consistent objective aspects and human minds perceive them the same way. A fifth interval in a chord has a frequency ratio of approx 3:2, thus it has a strong sound and the vast majority of listeners would agree. A minor second interval has a ratio of about 25:24 (even crazier, really), so it sounds harsh. 99.99999% of people will agree. Now, which harmonies are too harsh or confusing to be used in good taste has changed over the centuries, but their relative subjective quality remains the same. Objectively, a minor second and fifth have the same quality - none. But subjectively, they have a very different effect. Same thing for melodic intervals, the conjunct-ness of a melody, functional (i.e. I-IV-V) harmonic progressions, effects of rhythm, the logical effect of structuring and arrangement, how bizarre microtonal music sounds, etc. I don't want to go into a whole lecture on this, so just look it up for more info.
The second would be cultural consensus. I've read that different keys were once thought to have different characteristics. C minor was the key of heroic struggle, Bflat minor was morose and lonely, D major was celebratory, A major was suitable for music about love and so on. I don't think there is a psychoacoustic explanation for these, except that C major (the white keys of a piano) was considered the most basic key for a long time, so the farther a piece of music modulated from that, the stranger it would sound from what listeners were used to. Certain melodic and rhythmic patterns have become understood to have certain meanings, and some structures we get used to, and are exploited (see verse/chorus). Included in this is the practices of sticking to convention and breaking convention. Good pieces of music often seem to do both. There is an agreed-upon standard that we all understand, and they it is broken away from temporarily for effect.
Taking these small things into account, we recognize that all complex systems (the human mind, a computer, a symphony, or the human mind while listening to a simple song) are built out of small parts and the function of the large system depends on how these small elements interact. Some things are still to complex to compare their every element for an objective assessment which is better from any subjective point-of-view, but it could eventually be possible to do so, say with a thoroughly researched model of the average human mind and a big enough computer. <- Thinking about this, I wonder if it would then be possible for a computer to write The Perfect Song, or something close.
Put simply, you could say music has certain objective aspects that have strong subjective implications.